It's been long recognized that exercise and physical activity are crucial components of physical fitness and overall health and wellness. A well-rounded fitness program includes strength training to improve bone and joint function, bone density, muscle, tendon and ligament strength, as well as aerobic exercise to improve heart and lung fitness, flexibility, and balance. Resistance and weight training involve the use of resistance to muscular contraction to build strength and anaerobic endurance of skeletal muscles. In addition to the use of one's own body weight, many different types of equipment have been conceived for facilitating aerobic and anaerobic exercises, such as free weights, weight machines, medicine balls, and resistance bands.